xi 



It is impossible to read this comprehensive memoir without being 

 impressed by the boldness and sagacity of the author's genius, or 

 without admiring the simplicity of system he has introduced into a 

 subject, at first sight unapproachable from its difficulties. At the 

 same time, we must remember that the hypothesis on which he 

 builds is a particular one, and could only be accepted on its being 

 found that the conclusions are borne out by facts. Now the law that 

 the viscosity varies as the absolute temperature has not been verified 

 by other experimenters on this subject. It has been found that the 

 law of variation is a power of the temperature somewhat lower than 

 the first. But though we cannot, therefore, accept the author's con- 

 clusions without qualification, we must still regard his investigation 

 as a brilliant and determined effort to reach the true theory to which 

 it must form a close approximation. 



The methods employed by Maxwell were afterwards generalised by 

 Boltzmann, who succeeded in so modifying the proofs of many of the 

 theorems as to make them independent of any hypothesis as to the 

 nature of the encounter between the molecules. Maxwell to some 

 extent adopted Boltzmann's more general treatment of the theory in 

 his next memoir (1878) " On Stresses in Rarefied Gases arising from 

 Inequalities of Temperature ;" but he ultimately found himself com- 

 pelled to fall back on his former special hypothesis. In this memoir 

 he aims at an explanation of the forces producing motion in Crookes's 

 Radiometer. 



The statistical computation of the motion of a gas deprived of 

 that regularity which uniformity of temperature permitted is by 

 no means easy, and accordingly the work forms an elaborate appli- 

 cation of the principles developed in the paper of 1866. It is 

 shown, however, that the effect of inequality of temperature is to 

 produce a stress in the gas whose components are comparable with 

 the forces necessary to produce the motion of the radiometer wheel. 

 If we take account only of the normal components of this stress it is 

 possible to frame an explanation of that motion by their agency. 

 When, however, the general equations of the motion of the gas are also 

 determined, it appears that so long as the flow of heat is steady the 

 equations are the same as when the temperature is uniform. "How, 

 then, are we to account for the fact that forces act between solid 

 bodies immersed in rarefied gases, and this apparently as long as in- 

 equalities of temperature are maintained ?" The explanation, Maxwell 

 thinks, is to be found in the fact that the gas in contact with the 

 surface of a solid must slide over it with a finite velocity in order to 

 produce a finite tangential stress. This he is not able to show, but in 

 an appendix to the memoir added in May, 1879, the conditions which 

 must be satisfied at the boundary between a solid and a gas are inves- 

 tigated in a very ingenious manner, the results showing that " a gas 



